In completing wells in poorly consolidated or unconsolidated formations, consideration must be given to sand problems likely to arise during the operation of the well. The incompetent nature of this type of formation requires that the well completion technique include means for excluding sand production. Erosion and plugging effects of sand entrained in produced fluids are well known and if not arrested can seriously reduce well productivity.
The propensity of a formation to produce sand is particularly acute in formations characterized as unconsolidated or poorly consolidated. These terms, as applied to subterranean sedimentary deposits, define a particular class of sedimentary rock, the distinguishing characteristic of which is the absence of a rock matrix made up of sand grains bound into a cohesive mass.
Analytical model studies have indicated that unconsolidated rock can also be distinguished on the basis of mechanical behavior when subjected to loading. The injection of a viscous fluid into an unconsolidated body at high pressures causes the body to deform in a plastic-like manner. Field tests have shown that when subjected to fracturing pressures, subterranean unconsolidated formations deform in a plastic manner. In each of the wells treated the volumes of sand injected were considerably higher than that normally used in fracturing a consolidated formation. Moreover, the injection pressure for plastically deforming an unconsolidated formation was frequently higher than that normally required to fracture a consolidated formation.
A widely used sand control technique is the gravel pack installation which operates on the principle of forming a sand exclusion zone in surrounding relation to the wellbore. The sand exclusion zone composed of particularly graded aggregate screens out or bridges the formation sand entrained in the produced fluids. The typical gravel pack completion involves the placement of aggregate in the immediate vicinity of the wellbore and the provision of a support means for maintaining the aggregate in place. The aggregate is generally a specially graded sand or gravel, but can be other particulate material such as walnut shells, glass beads, and the like.
The placement of the aggregate immediately adjacent to the producing formation presents a major source of trouble in performing the gravel pack installation. Packing gravel into perforations is usually achieved by squeeze packing. Squeeze gravel packing is a two-stage process which includes (1) transportation of gravel slurry down through the pipe; and (2) gravel pack buildup at the formation face as fluid leaks off through the formation. In gravel packing of heterogeneous pays, frequently those perforations penetrating lower permeability formation are at best partially packed. This poor packing is caused by non-uniform leak-off rates through the heterogeneous formation. For a Newtonian carrier medium, Darcy's law states that the rate is directly proportional to the permeability. Carrier media used previously include low viscosity brine or fresh water, high viscosity oil, surfactant-base viscoelastic fluid, and shear-thinning fluid, e.g., hydroxyethylcellulose and xanthan gum solutions (HEC and XC solutions). Breakers were also added to degrade the fluid viscosity and enhance leak-off. However, the breaker has no improvement on gravel packing heterogeneous pays.
Therefore, what is needed is a carrier medium of fluid which has a different rheological behavior from those previously used so as to improve both transporting and packing efficiencies/uniformities.